<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta http-equiv="X-UA-Compatible" content="IE=edge">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>Document</title>
    <script src="js/vue.js"></script>
</head>

<body>
    <div id="app">
        km: <input type="number" v-model="km" />
        m: <input type="number" v-model="m" />
    </div>
    <script>
        var vm = new Vue({
                el: "#app",
                data: {
                    km: 0,
                    m: 0
                },
                methods: {
                    kmtom: function (val) {
                        this.m = this.km * 1000;
                    },
                    mtokm: function (val) {
                        this.km = this.m / 1000;
                    }
                },
                watch: {
                    km: 'kmtom',
                    m: 'mtokm'
                }
            })
    </script>
</body>

</html>